Reference signal design is an important issue to fully exploit the potential gain of the single carrier frequency division multiple access (SC-FDMA) system considered in the uplink of the evolved-UTRA (E-UTRA). The uplink is from user equipment (UE) to a node B. The UE may also be referred to as a mobile unit, mobile station, etc. The UE may be a wireless phone, wireless equipped PDA, a wireless equipped computer, etc. The node B may also be referred to as a base station, base station controller, base station router, etc.
The reference signal sent by a UE to the node B is needed for uplink CQI estimation as well as detection and coherent data demodulation. There has been on-going discussion on the uplink reference or pilot signal structure, and in particular, the pros and cons of code division multiplexing (CDM) and frequency division multiplexing (FDM) pilot structures. FDM involves dividing the bandwidth for sending reference signals into tones or sub-carriers and assigning, for example, different sub-carriers to different UEs. Each UE uses the same pilot sequence, but transmits that pilot sequence over different sub-carriers (i.e., different frequency or bandwidth). Code division multiplexing involves allowing each UE to use the entire bandwidth, but having each UE transmit using a differentiating pilot sequence. Well-known constant amplitude zero autocorrelation sequences (CAZAC) have been proposed as the codes. More specifically, the use of a generalized chirp like (GCL) sequence like a Zadoff-Chu sequence has been proposed. In these proposals, a GCL sequence spanning the bandwidth is chosen. A shifted version of the same GCL sequence is then assigned to each UE to differentiate between UE transmissions.
Desired elements of the SC-FDMA pilot signal design include:                1. Equal channel sounding in the frequency domain        2. Immunity to co-channel interference        3. Support of multiple user resource block sizes        4. Support of both localized and distributed sub-carrier mapping with reliable channel estimation performance        5. Efficient transmitter and receiver structures        6. Large number of sequences with the desired characteristic to support multi-cell deployment        
The FDM pilot has been advocated because it offers in-cell user orthogonality in the presence of fading. So far, comb-shaped pilots and staggered pilots are the proposed reference patterns for the FDM pilot. One of the major drawbacks of FDM pilots is the impact of dominant co-channel interference. When two users at the cell edge use the same pilot sub-carriers, the channel cannot be estimated reliably due to collision.